A proving device for proving a flowmeter is a device for subjecting a newly manufactured flowmeter or a flowmeter in use to a characteristic test periodically or at any timing in order to use the flowmeter at a reliable accuracy regardless of a change in the characteristics attributable to external factors such as temperature and pressure or to internal factors such as abrasion of moving parts. Broadly classified, the characteristic test is carried out by two methods, one using a calibrator that sets a flowmeter to be test in a fixed tester to perform the test and the other using a prover type flowmeter tester that sets the flowmeter in a fluid system to optionally perform the test.
Since the prover type flowmeter tester can perform the flowmeter characteristic test on line and perform any characteristic test as needed, it is often used in the test of an inferential flowmeter especially susceptible to piping, e.g. of a turbine meter. The prover is a device having a moving element such as a piston that moves through a pipe with a constant section in synchronism with fluid and using as a reference volume a fluid displaced by the movement of the moving element through a predetermined distance.
In the flowmeter characteristic test using the prover type flowmeter tester, the number of flow pulses sent per unit volume (flow rate coefficient), i.e., a so-called K factor is calculated from the reading of the flowmeter when a fluid of a reference volume specified by the prover passes therethrough, that is, by detecting the number of flow pulses sent from the flowmeter when a reference volume of fluid passes therethrough. If necessary, a continuous flow rate characteristic curve is obtained based on the flow rate coefficients in a plurality of flow rates to be measured.
To obtain the flow rate coefficient at a high resolution, the number of flow pulses sent per reference volume needs to be a predetermined number or more and, for example, a specification of 10,000 pulses or more is given to a large-sized stationary prover having a large reference volume. On the contrary, if the reference volume is reduced, more than a specified number of flow pulses cannot be issued, but the flow rate coefficient can be obtained from the relationship between the reference volume of a fluid displaced as a result of movement of the moving element such as the piston and the issued pulses (time) issued from the flowmeter during the movement. Accordingly, regardless of the reduced number of flow pulses, a small-sized prover (small volume prover) is available.
In the small volume prover (hereinafter, referred to as SVP), a piston prover is known that uses the piston as the moving element. The piston prover has a measurement cylinder with a constant section that is basically connected in series with a flowmeter to be tested and compares the volume of a fluid displaced when the piston moving in the measurement cylinder moves through certain distance with a reading of the flowmeter at that time. The volume of a fluid is actually obtained from the amount of movement of the piston. In the proving, ordinarily, plural times of test results are averaged to figure out the flow rate coefficient (K factor) based on the average value. Consequently, the piston reciprocates by the number of times of the test in the measurement cylinder for each of the flow rates to be measured.
To return the piston to its original position after the completion of the measurement by the movement of the piston through a specified distance in the measurement cylinder, the piston is driven against the flow of fluid by an actuator using hydraulic pressure or pneumatic pressure via a piston rod. A flow passage allowing a fluid to pass therethrough during the drive is provided by two manners, one using the measurement cylinder itself and the other using a bypass flow passage separately disposed in parallel with the measurement cylinder. In case of causing a fluid to pass through the measurement cylinder, a valve function is provided in the interior of the piston returned by the actuator such that the valve is closed at the time of the measurement and that it is opened in the return stroke. This method is called an internal valve method. In case of causing a fluid to pass through the bypass flow passage, a bypass valve is disposed in the bypass flow passage such that the valve is closed at the time of the measurement and that it is opened in the return stroke. This method is called an external valve method.
Known as such an SVP is one described in Patent Document 1 for example. To form the reference volume, the SVP is configured such that a detector switch detects a position of a marker attached to the piston rod secured to the measuring piston. Accordingly, invar (alloy having a low coefficient of thermal expansion) of the piston rod is grooved or processing such as ceramic coating is added to the entire invar. When returning the measuring piston to a predetermined upstream position, a hydraulic pressure is applied to an end face of the piston rod secured to the measuring piston to move the measuring piston toward upstream. A separately designed slide valve (piston valve) is used for the flow passage switching.